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A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

Seven years ago this week I was preparing one of my favorite lectures for The Formation and Evolution of Planetary Systems, a class I frequently teach at Caltech. “Preparing” is probably the wrong word here, because this lecture, called The Edge of the Solar System, was one I could give even if instantly wakened from a cold deep sleep and immediately put on stage with bright lights in my eyes and an audience of thousands and no coffee anywhere in sight. The lecture explored what was known about the edge of our main planetary system and the ragged belt of debris called the Kuiper belt that quickly faded to empty space not that much beyond Neptune. Conveniently, one of my most active areas of research at that time was trying to figure out precisely why this ragged belt of debris had such an edge to it and why there appeared to be nothing at all beyond that edge. I could wing it. So instead of preparing the lecture, I really spent that morning doing what I did whenever I had a few spare moments: staring at dozens of little postage-stamp cutouts of pictures of the sky that my telescope had taken the night before and my computer had flagged as potentially interesting. Interesting, to my computer, and to me, meant that in the middle of the postage stamp was something that was moving across the sky at the right rate to mark it as part of the Kuiper belt. I was not just lecturing about this debris at the edge of the solar system, I was looking for more of it, too.

I didn’t find more objects in the Kuiper belt every morning I looked, but that previous night seven years ago had been a good one. I quickly found two of the typical debris chunks moving slowly across the sky, and I was about ready to walk over to give my lecture, when, with only about a minute to spare, the outer solar system seemed to change before my eyes.

There, on my computer screen, was a faint object moving so slowly it could only have been something far more distant than what I was just going to walk into the classroom and declare to be the edge of the solar system. Maybe. The object was so faint that I didn’t know whether to believe it was real or not. If you look at enough sky – and, really, I had – you are bound to find some chance alignment of blips of noise or variable stars or cat hairs that looks just like something real.

I went into the classroom, delivered the lecture as I knew it, but stopped short at the end.

“Here is the way I was going to end this lecture,” I told them.

I proceeded to talk about how nothing existed beyond the edge of the Kuiper belt (yes, yes, you sticklers, the Oort cloud is way out there, but that is not supposed to start up until 100 or 200 times further out than the edge of the Kuiper belt).

“But I’m not sure I believe this anymore,” I said.

I told them about that morning’s blip. I couldn’t promise them that it was real, but I told them that if it was, the solar system might be very different place than I was just telling them.

That little blip, far more distant than what was supposed to have been the edge of the solar system, was indeed real. It was Sedna.

Sedna is the Inuit goddess of the sea, often depicted with the body of a seal, long hair, and no fingers.

A few weeks later, after confirming that Sedna was real and determining its unprecedentedly strange orbit around the sun, I came back, told the class all about it, and wrote down a few simple equations on the blackboard to show just how strange the orbit is and also the many different ways it might have gotten that way.

“Come back and take my class again next year, and I’ll have it all figured out,” I confidently told them.

That was seven years ago. Any poor student taking my advice would have sat through the last six years of lectures and still not learned what put Sedna where it is, since I still don’t know the answer.

What makes Sedna’s orbit so strange?

Sedna takes 12,000 years to go around the sun on its elongated orbit, and it never comes close to any of the planets.

Many objects out in the Kuiper belt have shockingly elongated orbits like Sedna. For almost all of these objects, this characteristic makes sense. These small leftover pieces of debris have been kicked around by planets throughout their existence. Whenever they come too close to one of the planets (usually Neptune, since it is the closest to these objects), they get a gravitational kick that can send them on a looping orbit to the distant outskirts of the solar system. But – and this is the key part here – unless they get kicked all the way out of the solar system, they always come back to where they were kicked. If you get kicked by Neptune, you can go zooming off into the unchartered regions far beyond the Kuiper belt, but you will come back to see Neptune again. When we look at the Kuiper belt, we see the results of all of this kicking clearly: the Kuiper belt objects that come closest to Neptune are on the most elongated orbits. Those far away are more free to go about their circular orbiting lives.

The exception to this rule is, of course, Sedna. Sedna has one of the most elongated orbits around, but it never comes anywhere close to Neptune or to any other planet. Indeed, the earth comes closer to Neptune than Sedna ever does. And the earth is not in danger of being kicked out of its orbit by Neptune anytime soon.

Something had to have kicked Sedna to have given it its crazy orbit. But what?

The answer is: something large that is no longer there, or that is there, but we don’t know about yet.

This answer is astounding. The orbit of every single other object in the entire solar system can be explained, at least in principle, by some interaction with the known planets (and, again, for you Oort cloud sticklers out there, the known galactic environment). Sedna alone requires Something Else Out There.

What is it? Seven years out, we still don’t know. The hypothesized culprits have included passing stars, hidden planets, Oort cloud brown dwarfs, and, of course, Sumerian-inspired alien conspiracy theories. Whatever it is, it is bound to answer profound questions about the origin and evolution of the solar system, as well as inspire many new questions we had never known to ask.

@Maju -- Ha! I hope I reveal something else next week too because that would mean I would have learned the answer between now and then. But, barring that, I'll just be talking about the process of science and how we're **trying** to find out the answer. Frustrating? Oh yeah.

You write: "But – and this is the key part here – unless they get kicked all the way out of the solar system, they always come back to where they were kicked."

Question: Is it known whether such dwarf planets (and presumably comets and asteroids) do sometimes get kicked out of the solar system altogether? And if so, presumably there are "free" planetoids wandering out there in interstellar space. How many would there be? Is it conceivable Sedna might have been such a body, that somehow got captured by the sun's gravity, interacting with that of Sedna and some other non-solar system body that has since moved on?

As an aside, it is nice to see that new bodies are nowadays named after other mythologies than western ones. But the name "Sedna" always makes me think of those ladies with 1950s glasses that Gary Larson used to draw in his surreal cartoons... ;-)

@Brian -- Bonus points for careful reading. You are Abso-100%-lutely right. No one has a good idea of how many such interstellar travelers there might be, but we KNOW that they are out there. And, yeah, Sedna could be one of them. I think it's a long shot, but it's not something that I can yet rule out!

Sedna has got but also hasn't got crazy orbit. This planetoid was discovered on one of the places what were suitable for possible position of X in that time (2003), based on ancient sources like Senmut map,funeral banner of markiza Tai, ancient carpet,...Maybe Sedna was satelite of X,...Direction Orion is place with strongest gravit. tug from closest stars (Siriuses, Procyons,..)and mainly from nebula M42. M42 is birth place of stars, planets, planetoids what could been captured also by our Sun. Sedna orbits circa on the line direction Orion,Sun,...more on my http://senmut.webs.com/sednaandplanetoids.htmI think too that Oort belt is much more closer how it was foretold, that source of those long periodic comets is circa 1000AU from us, where dark brown dwarf or something other small, dark but heavy with mass approx. 30Jupiters is orbiting around Sun, with period cca 26000years, what is also precession time of Earth,..this way it is also depicted on ancient astronomical carpet,..X/dark, small, but massive (5-25Jupiters) is hurtling toward us with perihelia in 2012?!Who will observe it first? Or it was observed yet but considered only for asteroid,.. due to it's small dimensions,..http://senmut.webs.com/apps/blog/Pavel

@Pavel Of course, the idea that ancient funeral banners etc. give any insight into Sedna or a planet X or anything about 2012 puts this firmly into the category of pseudo-science, rather than science Does this make it wrong? Not necessarily. Sometimes myths and fables contain a germ of truth to them. How will we ever know? Easiest way is to stick around until 2013, note that no massive object recently flew past us, and remind ourselves that science does a good job of making these sorts of predictions and that pseudo-science, while fascinating to speculate about, should not be used to try to predict reality.

Nemesis? There was a speculation at one time that there was a red dwarf star (80+ Jupiter masses) that caused periodic mass extinctions on Earth. The people who thought of that idea hadn't looked at the list of known asteroids. #128 is Nemesis.

Whitmire has a location for the object by backtracking long-period comets. Assuming that he's right and it exists, it may or may not be the Nemesis object, it might just be a passing star. He says it's 1-4 Jupiter masses.

I put it on GravitySimulator to see how big an object needs to be to cause the observed chaos in the Kuiper belt and got 5 - 25 Jupiter masses, but no RA or Dec.

The Widefield Infrared Survey Explorer (WISE) is out looking for it now. Sedna's the best evidence that WISE will find something.

Eris is one of those way-out objects. Neptune can kick Eris out to the distance it's at, but it cannot get the high inclination. I've tried everything. Neptune can cause up to about 30 degrees inclination; after that, any encounter with Neptune decreases inclination and increases eccentricity. It's called the Kozai mechanism. The square root of (1 - e^2) times cos i is a constant, where e is the eccentricity of the orbit and i is the inclination.

http://en.wikipedia.org/wiki/Kozai_mechanism

The only thing that is going to overcome the Kozai mechanism is the something that is out there, and it needs to be pretty big. So, Eris also says there is something out there.

We're lucky to see Sedna. During nearly all of it's orbit it is much too far away to see. Since we DO see it it makes sense that it's just one of a class of several objects similar to it.

If Sedna is an escaped planet of Tyche then Tyche must at one time have had a large amount of material around it compared to the Sun or Jupiter or Neptune. Very interesting.

"I love a mystery..." I'm quessing Sedna was dropped down into it's current orbit from a nearby system in the sun's formation cluster?Fascinating! I definitely think we should send out a probe some day before it gets too far out.

Since Sedna has a 12,000 year period, I don't think it's getting away very fast. :-) If we sent a probe out there within the next 50 years, I think that'd be about the same as sending one now.

Trouble is, with our current technology, I'd say it's already too far out. NH is taking 9 years to reach Pluto, so one would estimate a similar probe would take 30 years to reach Sedna. It sure would suck to be the PI on that mission!

Thanks, informative as well as provoking. (And I assume from the twitter handle that is how you like them. :-D)

@ mikeemmert:"There was a speculation at one time that there was a red dwarf star (80+ Jupiter masses) that caused periodic mass extinctions on Earth."

Which hypothesis was originated in one of those papers that mashed statistics, and not surprisingly any putative effect is now rejected (by proper auto-correlation studies) with better stats in the fossil record. (I don't have the ref handy, but can provide if asked.)

If there is anything massive still out there, we can't notice it here on Earth, no impactor iridium (AFAIK) nor extinctions. It's a long shot, and that isn't because it is far out there (or perhaps it is considering the evidence at hand).

Why would'nt you consider that it was "kicked" by another asteroid or comet? Sort of the old "billiard ball phenomenon". Seems like this would be a logical answer before some of these far fetched thoughts.

My 3 year old son is entranced by your stories of Sedna and all the talk of objects kicking it into its elongated path. As am I. Since this was posted about 6 years ago, I'm really hoping there's even more information with plausible theories of its orbit. Where can I find more?